Because of the large inherent bandwidth of optical systems, such systems are being increasingly utilized for transmitting information, generally over fiber optic cables, in telephone, cable television, data transmission and processing and other applications. Heretofore, such systems have typically used amplitude modulation (AM) because it is simple to implement. However, frequency modulation (FM) is more power efficient as compared with amplitude modulation and can provide an order of magnitude or more increase in signal-to-noise ratio. While this signal-to-noise ratio advantage has been taken advantage of for many years in radio, FM is not widely employed to any extent in optical communications. Thus, even though wireless transmission encodes signals via FM because FM offers substantial power savings over AM, optical transmission tends towards AM because component technology for FM receivers and sources is limited.
For analog communication or communication involving a multiplicity of discrete optical frequency levels, where noise may present problems, current optical frequency discriminators are typically non-linear and have been found to provide linearity over only such a short frequency range that it is difficult to obtain distortion free outputs over a useful range of modulating frequencies. The lack of an optical frequency discriminator which provides a substantially linear output over an extended frequency range still presents a problem.
A need therefore exists for an improved frequency discriminator for use in optical FM communication systems and other applications, which provides a substantially linear output over a selected bandwidth.